Conventionally, industrial wastewaters containing organic matter from various industries have been treated by biological treatments such as the standard activated sludge method. The standard activated sludge method is a biological treatment method which treats organic wastewater aerobically. FIG. 7 is a flow chart showing the standard activated sludge method. Incoming wastewater is fed into an activated sludge tank 101, and the organic matter (phenol, benzene, etc.), nitrogen, phosphorus, and others therein are biologically treated with activated sludge, which is a group of various microbes. Then, after sedimentation of the activated sludge in a sedimentation tank 102, the excess sludge is withdrawn and the supernatant liquid is obtained as the treated water. Although organic substances could be treated by the method, the quality (clarity) of the treated water was insufficient and thus the treated water was underqualified for being recycled, for example, as industrial water.
Recently, on the other hand, the so-called membrane-separation activated sludge method or membrane bioreactor (MBR) method by using ultrafiltration membrane has been employed for treatment of sewage and excrement, and the treated water is recycled in some cases. FIG. 8 is a flow chart showing the membrane-separation activated sludge method (membrane bioreactor (MBR) method). Incoming wastewater is fed into an activated sludge tank 101 for biological treatment of the organic matter (phenol, benzene, etc.), nitrogen, phosphorus and others with activated sludge which is a group of various microbes. Then, the wastewater is separated from the solids and concentrated by filtration with a membrane module 103 to give filtered water. The filtered water is clear and recyclable. In addition, the membrane-separation activated sludge method has many advantages over the standard activated sludge method in that the treatment can be carried out in a compact area, maintenance and control are easier, and the system is resistant to fluctuations in load (e.g., see JP-B-6-34999 (“JP-B” means examined Japanese patent publication)).
However, there is almost no known example of application of the membrane-separation activated sludge method to industrial wastewaters such as those from petrochemical and petroleum refining plants.
One of the reasons for that is that industrial wastewaters, especially those from petrochemical and petroleum refining plants, often contain organic compounds that change into highly toxic substances in a reaction with chlorine, though the membrane-separation activated sludge method demands periodical injection of chlorine or a chlorine compound (a chlorine-based chemical) as the membrane-cleaning agent for sterilization (cleaning) of the internal and external surface of the membrane in membrane module, so that the highly toxic substances are produced when washing the membrane. The membrane-separation activated sludge method commonly used is an immersion-type membrane-separation activated sludge method of immersing a membrane module 104, as shown in FIG. 9, in a sludge tank (aeration tank) 101 and filtering the wastewater by suction with a suction pump 105, thereby obtaining the filtered water. In such an immersion-type membrane-separation activated sludge method, the valve 106 is closed during membrane cleaning and, normally, a chlorine-based chemical is supplied into the membrane module 104, as indicated by an arrow in the FIG. 9. There was some concern then about the generation of toxic chlorinated organic compounds such as chlorophenol 108, in a reaction of chlorine with the organic compounds such as phenol 107 contained biologically untreated in the wastewater. Because chlorophenol is a compound with a noticeable foreign odor, even if present in a trace amount, thus, the presence of the chlorophenol in the filtered water, even in a trace amount, prohibits recycling of the filtered water significantly.
The membrane-separation activated sludge method demands periodical chlorination by using a chlorine-based chemical but, disadvantageously, in the wastewater to be treated, there are often metals, such as manganese, that are oxidized in the presence of chlorine to metal compounds that precipitate on the surface or in the membrane of the membrane module. The metal compounds precipitating on the surface of or in the membrane of membrane module may inhibit the operation of the wastewater treatment facility and may also cause the same problem of metal compound precipitation in a reaction with an alkali agent contained in the membrane-cleaning agent.
In wastewater treatment to which the membrane-separation activated sludge method is applied, the membrane should be cleaned periodically with a membrane-cleaning agent, and there are cases where the wastewater to be treated contains substances generating hazardous substances and substances inhibiting a treatment process in a reaction with the membrane-cleaning agent. The inhibition of the treatment process above means the deposition of operation-inhibiting substances on the membrane surface or precipitation thereof in the membrane, clogging the membrane and inhibiting membrane separation.
The present invention is related, for example, to a process of treating a wastewater containing substances generating chlorine-based toxic compounds in reaction with the chlorine-based chemical normally used as the membrane-cleaning agent. Examples of hazardous compound-generating substances include activated aromatic compounds such as phenol and aniline and carbonyl compounds such as aldehydes and ketones. Phenol, one of the compounds above, generates toxic chlorophenol with a strong odor in reaction with chlorine.
Further, if metals (metal ions) are contained in the wastewater, the metals may form, in a reaction, for example, with chlorine or an alkali agent in the membrane-cleaning agent, metal compounds that deposit on the surface of the membrane of membrane module and precipitate inside the membrane clogging the membrane and thus reducing filtration capacity significantly. For example, manganese is oxidized in the presence of chlorine, giving a precipitate of hydrated manganese dioxide (MnO2.mH2O), and leading to the deposition thereof on the membrane surface, precipitation inside the membrane, clogging the membrane and reducing the filtration capacity. In addition, lead precipitates as PbCl2 in a reaction with hydrochloric acid.
Metals such as iron and manganese are drastically less soluble in the alkali-side region of pH and, thus, alkali cleaning immediately after acid cleaning highly possibly results in the precipitation of the metal compounds on the membrane surface and inside the membrane.
Thus, there is a demand for the prevention of precipitation of such metal compounds on the surface of and in the membrane of the membrane module.
In comparison with internal-tank membrane modules, external-tank membrane modules allow the use of a membrane-cleaning agent at higher concentrations (to the degree permitted by the membrane material) without an influence on the in-tank microbes, even if it is a bactericide-based agent, an alkali-based agent, or an acid-based agent, but use of such a membrane-cleaning agent at a high concentration may possibly enhance the concern about the precipitation of by-products and metal compounds.